Painful diabetic neuropathy is the most common form of painful neuropathy and causes great individual suffering and expense to society. While symptomatic therapy, for some, can provide modest relief, in most patients pain is poorly controlled. The development of successful therapy has been hampered by lack of an understanding of the cellular mechanisms in the primary afferent nociceptor underlying in this pain. We propose to perform multi-disciplinary in vivo and in vitro investigations in a rat model of painful diabetic neuropathy, the streptozotocin-induced diabetic (STZ-D) rat, to first identify the electrophysiological abnormalities in nociceptive nerve fibers, and then to determine second messenger pathways involved in producing these abnormalities. STZ-D rats exhibit hyperalgesia characterized by decreased thresholds for nociceptive paw-withdrawal reflexes, and increased activity in C-fiber nociceptors in sensory nerves. We have provided evidence that both the cAMP/protein kinase A (PKA) and protein kinase C (PKC) second messenger systems contribute to diabetic hyperalgesia. Guided by these findings we will perform a detailed analysis of the features of the C-fiber hyperexcitability. In addition, we will perform a detailed study of the contributions of the PKA and PKC pathways in producing these identified features of diabetes-induced nociceptor hyperexcitability as well as in causing the behavior phenomenon of diabetic hyperalgesia. Finally, we will evaluate the role of increased glucose levels in causing these abnormalities. The goal of these experiments is to obtain novel information about diabetes induced hyper-algesia, nerve fiber hyperresponsiveness, and second messenger mechanisms in order to develop an integrated, coherent mechanistic model of painful diabetic neuropathy. Such a model could provide a foundation for the development of rational therapies for the treatment of painful diabetic neuropathy in human.